The present invention relates to the art of post-treating conversion-coated zinc or zinc alloy surfaces to improve the corrosion resistance, paint receptivity and other qualities of the surface. More specifically, it relates to the art of accomplishing the foregoing without employing objectionable chromium chemicals.
Zinc and zinc alloys have been conventionally applied to steel surfaces by the electroplating or hot galvanizing process in order to improve corrosion resistance. The galvanized steel plate prepared by such methods have been conversion coated (e.g., chromated or phosphated) and/or oiled to further enhance corrosion resistance. There are also galvanized steel plates which have been first phosphated, compound oxide coated (e.g., See U.S. Pat. Nos. 2,762,733 and 2,849,344), or chromated and then painted and cured.
Chromates are conventionally employed not only in the chromating process but as post-treatments in the phosphating and compound oxide coating processes. The anti-corrosion effect of the chromating treatment on metals, especially on steel, zinc or aluminum is outstanding and consequently chromates have been widely employed in the field of metal surface treatment. Nevertheless, the environmental and health hazards of the toxic chromium compounds have come into question, and it has become an objective to develop safer surface treating agents which can be employed in place of chromium.
Various studies have been made concerning treating compositions containing no chromates. Processes for treating metal or conversion coated metal surfaces with compositions containing mainly phytic acid are suggested in Japanese patent publication Nos. 23462/1963, 11006/1969, 21566/1970 and 43406/1973 and Prepublished Japanese patent application No. 104845/1974. Treating with compositions containing mainly alpha-aminophosphonic acid or alphaaminophosphonous acid is suggested in Prepublished Japanese patent application No. 78531/1975, and treating with compositions containing mainly tannic acid is suggested in Prepublished Japanese patent application No. 47224/1974. Such processes, however, are unsatisfactory when employed for practical uses on zinc or alloys thereof.
The following are conventional treatment cycles:
(1) Phosphating PA0 (2) Compound oxide coating process:
(a) Hot galvanized plates PA1 (b) Electrogavanized plates PA1 (ii) Hot water rinse.
(i) Wet buffing if the surface has been chromated. PA2 (ii) Hot water rinse. PA2 (iii) Surface conditioning with colloidal titanium phosphate PA2 (iv) Phosphating PA2 (v) Hot water rinse. PA2 (vi) Post treatment. PA2 (vii) Dry. PA2 (i) Cleaning with a weak alkali (this alkali often contains colloidal titanium phosphate to effect concurrently the surface conditioning) PA2 (iii) Phosphating PA2 (iv) Hot water rinse. PA2 (v) Post-treatment. PA2 (vi) Dry. PA2 (i) Alkali cleaning. PA2 (ii) Wet buffing if the surface has been chromated. PA2 (iii) Hot water rinse. PA2 (iv) Compound oxide formation. PA2 (v) Hot water rinse. PA2 (vi) Post-treatment PA2 (vii) Dry.